This invention relates generally to internal osteosynthesis plates for fracture or reconstructive repair of long bones in patients, such as the humerus, radius, ulna, tibia and femur. A common principle in long-bone fracture management is interfragmentary contact and stabilization until sufficient osseous healing occurs to mobilize the fracture. During this period, an adequate healing response relies on the combination of bone-to-bone contact and plate fixation to obtain the optimum load sharing distribution across the reparative site. However, in contrast to fracture repair, there are clinical situations requiring major long-bone reconstruction; as in leg-length inequality, major reconstruction after trauma, or following radical tumor resection, wherein cortical bone-to-bone contact is initially absent at the reparative site and must be repaired by bone grafts. These latter bone graft cases, by their very nature, preclude the important biomechanical advantage of early load sharing offered by cortical bone contact. In these instances, any load transfer must be through the plate and associated plate screws until adequate osseous consolidation allows for functional load sharing between bone segments and the internal fixation device.
In fracture fixation or reconstructive procedures, a bony discontinuity, be it a fracture with or without adequate reduction, or a bone gap, as in reconstructive procedures, must be suitably stabilized and provide sufficient load transfer. Although it has been widely reported that inadequate anatomic reduction during fracture fixation can result in excessively high plate stresses, which can lead to premature failure, little is known about the loads being simultaneously transferred through the individual plate screws. Furthermore, if bone-to-bone contact is not achieved in fracture reduction, or can not be achieved as in long-bone reconstruction, then even greater load demands must be placed upon individual plate screws.
During the past ten years, there has been a substantial increase in long-bone reconstruction surgery employing autogenous cancellous bone grafts where bone-to-bone contact can not be initially achieved. Because these bone gaps average five centimeters, osseous healing requires many months to consolidate sufficiently prior to full weight-bearing, thus imposing high, long-term loading demands upon the plate and screw fixation systems now in use. Further complicating an uneventful rehabilitation of these patients is the desire and need for early controlled ambulation, to expedite osseous healing, which results in additional loading of the plate-screw fixation system.
The standard leg-lengthening plate, currently in surgical use today, is essentially a broad or narrow fracture plate whose mid-section is simply elongated and devoid of screw holes in order to bridge a large bone graft site. Since a fracture plate is generally intended for use with cortical bone-to-bone contact, its design is unsuitable for use in surgical situations where bone contact is initially absent or unstable, as in reconstructive surgery involving large gap bone grafts. Therefore, a fracture plate with a simple elongated mid-section can not be relied upon to support early ambulation or unusual intermittent higher loads, since its inherent total stiffness, associated with asymmetric femoral loading, transfers significantly greater non-uniform bending strains to the plate screws. In the presence of asymmetric long-bone bending, excessive screw bending strains are produced and result in a high percentage of screw failures in these fixation regions, with subsequent loss of plate fixation. Analysis of radiographs have demonstrated that the sequence of femoral screw failure generally starts at the most proximal plate screw (i.e., the screw closest to the patient's pelvis at the plate tip) and proceeds distally towards the plate mid-section. Scanning electron microscopy studies of failed screw surfaces have been conducted and demonstrate that screw failure was predominantly the result of bending fatigue rather than shear failure.
The highly asymmetrical failure pattern of these plate screws--almost all proximal to the plate mid-section, in association with individual screws predominantly failing in bending--gave rise to the theory that asymmetrical femoral bending, during loading, was the cause of these clinical implant failures. Furthermore, any long-bone subjected to asymmetric bending during loading would demonstrate non-uniform screw strains if the fracture or fixation plate used is symmetrical in shape on either side of the reparative site. The significance of this invention is that the plate fixation design is more appropriately matched to the loading requirements of individual long-bones as opposed to plates designed to fit the anatomical contours of a bone or the site of reparative fixation.
To investigate further the mechanism of clinical failure, a single-stance anatomic model was employed by me to simulate the fixation mechanics of the then-current surgical leg-lengthening procedure. This analysis demonstrated that asymmetric loading of the femur could result in abnormally high screw bending strains leading to the screw bending fatigue seen clinically. Based on these studies, the operative procedure was modified by surgically positioning the conventional leg-lengthening plate as posteriorly as possible to counteract the large lateral bending moments produced along the proximal femoral shaft. To reduce further excessive bending strain levels on a per-screw basis, ten-hole plates (five screws on each side of the bone gap) were substituted for the normally used eight-hole plates (four screws on each side of the bone gap). Although the posteriorly-positioned, ten-hole plate did reduce proximal screw failures, its surgical placement was not easily accomplished, nor could it serve as a permanent solution in reducing excessive strains on the proximal plate screws. However, plate repositioning did reduce screw failures, thus providing clinical confirmation that excessively high bending loads were being applied to the proximal plate screws.
These research studies and associated clinical results clearly indicated the need to understand better the basic interrelationships involving the transfer of screw loads between plate and bone during osseous healing, both for long-term reconstruction and fracture repair. In fact, one may take the position that a dichotomy exists in observing two grossly different modes of implant failure for two relatively similar fixation conditions. For example, implant failure during delayed or non-unions of poorly reduced midshaft femoral fractures result primarily in plate failure, while in femoral reconstruction cases, broken screws are the primary cause of failure.
My studies have demonstrated that there are substantial differences in the transfer of screw loads during fracture repair as opposed to long-bone reconstruction, thus providing a unique insight into the mechanisms of plate fixation. Furthermore, these studies provided the basis for the development by me of the dual taper, asymmetric hole placement in reconstruction and fracture plates of the present invention.
It is an object of the present invention to create a reconstruction or fracture plate suitable for use in large-gap, long bone grafts, that is less susceptible to anchoring screw and plate failure than previously known reconstruction or fracture long bone plates, that eliminates medical complications caused by failure, such as potential loss of the patient's leg length; increased medical risks from delayed bone union; increased hospitalization and costs associated therewith; and additional surgery necessary to salvage the plate failure.
More particularly, it is an object of the present invention to create a reconstruction or fracture plate, having varying flexibility along the plate and an asymmetrical array of screw holes for receipt of anchoring screws, that distributes more equalized loads to each screw than previously known plates, while secondarily improving load transfer at the reparative site and reducing stress shielding.